Add README

README.md

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+# dm-era
+
+dm-era is a Device Mapper target that acts as a proxy to an existing block device, like dm-linear,
+but also keeps track of which blocks were written to. It is included in mainline Linux kernel since 3.15.
+
+# era_copy and era_apply
+
+era_copy parses dm-era metadata from `era_invalidate` output and saves changed blocks into a stream
+that you can save to a file or copy over network.
+
+era_apply takes era_copy output and applies it to a file (or to a block device) to create
+a mirror of the original device.
+
+With dm-era and these two small utilities you can perform incremental backups of raw block devices
+without the need for a COW FS, LVM or a storage hypervisor. dm-era almost doesn't hurt performance
+and seems to handle fsyncs correctly.
+
+# How to try dm-era for incremental backups (Debian, GRUB)
+
+1. Setup a small partition for dm-era metadata. We'll use 1024 block (512 KB) granularity
+   so a bitmap for 1 TB device will only take 8 MB on disk. dm-era keeps slightly more than one bitmap
+   on disk at a time, but anyway, a 512 MB or 1 GB partition will be more than enough.
+   For example you can shrink the main partition a bit and add the dm-era partition after it.
+2. Zero out the new metadata partition: `dd if=/dev/zero of=<META_PARTITION> bs=1048576`
+3. Copy `local-block_dm-era.sh` to `/etc/initramfs-tools/scripts/local-block`
+   and adjust `DATA_DEVICE`, `META_DEVICE` and `ERA_DEVICE_NAME` in it.
+   Use GPT and partition UUIDs to be safe because dm-era doesn't check
+   if you supply the correct partition to it.
+4. Edit `/etc/fstab` and change your actual device to `/dev/mapper/<ERA_DEVICE_NAME>`,
+   for example `/dev/mapper/root_era`.
+5. Repeat it for more partitions if you want.
+6. If you do it for the root partition also change `/etc/default/grub`:
+   `GRUB_CMDLINE_LINUX="root=/dev/mapper/root_era"` and refresh grub config with `update-grub`.
+7. Reboot.
+8. Install dm-era tools (`era_invalidate`) with `apt-get install thin-provisioning-tools` on your target host.
+9. Install `era_copy` and `era_apply` (`make install` from this repository) on both hosts (target host and backup host).
+10. Do an initial full partition backup with block-level copy.
+11. Now you can use `backup.sh` to perform incremental backups of the dm-era device
+    over ssh from the backup host. Just change variables at the top of the script so it matches
+    your device configuration.
+
+It's not a one-click solution, but it works :-)
+
+# Author and license
+
+Vitaliy Filippov
+
+GNU GPLv3.0 or later
+
+Of course I don't take any responsibility if you kill your data while trying to setup dm-era :-)

local-block_dm-era.sh

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+#!/bin/sh
+
+set -e
+
+PREREQ=""
+
+prereqs()
+{
+    echo "${PREREQ}"
+}
+
+case "${1}" in
+    prereqs)
+        prereqs
+        exit 0
+        ;;
+esac
+
+modprobe dm-era
+
+DATA_DEVICE=/dev/disk/by-partuuid/ca313f78-7911-994b-b465-5cda9a3ceafb
+META_DEVICE=/dev/disk/by-partuuid/be47b5e2-1587-1243-9c5f-f9e165df5c30
+ERA_DEVICE_NAME=root_era
+dmsetup create $ERA_DEVICE_NAME --table "0 `/sbin/blockdev --getsz $DATA_DEVICE` era $META_DEVICE $DATA_DEVICE 1024"